Concurrency is "lightweight", which means that both thread creation
and context switching overheads are extremely low. Scheduling of
Haskell threads is done internally in the Haskell runtime system, and
doesn't make use of any operating system-supplied thread packages.

However, if you want to interact with a foreign library that expects your
program to use the operating system-supplied thread package, you can do so
by using forkOS instead of forkIO.

Haskell threads can communicate via MVars, a kind of synchronised
mutable variable (see Control.Concurrent.MVar). Several common
concurrency abstractions can be built from MVars, and these are
provided by the Control.Concurrent library.
In GHC, threads may also communicate via exceptions.

A ThreadId is an abstract type representing a handle to a thread.
ThreadId is an instance of Eq, Ord and Show, where
the Ord instance implements an arbitrary total ordering over
ThreadIds. The Show instance lets you convert an arbitrary-valued
ThreadId to string form; showing a ThreadId value is occasionally
useful when debugging or diagnosing the behaviour of a concurrent
program.

Note: in GHC, if you have a ThreadId, you essentially have
a pointer to the thread itself. This means the thread itself can't be
garbage collected until you drop the ThreadId.
This misfeature will hopefully be corrected at a later date.

Note: Hugs does not provide any operations on other threads;
it defines ThreadId as a synonym for ().

killThread terminates the given thread (GHC only).
Any work already done by the thread isn't
lost: the computation is suspended until required by another thread.
The memory used by the thread will be garbage collected if it isn't
referenced from anywhere. The killThread function is defined in
terms of throwTo:

throwTo does not return until the exception has been raised in the
target thread. The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.

Scheduling may be either pre-emptive or co-operative,
depending on the implementation of Concurrent Haskell (see below
for imformation related to specific compilers). In a co-operative
system, context switches only occur when you use one of the
primitives defined in this module. This means that programs such
as:

will print either aaaaaaaaaaaaaa... or bbbbbbbbbbbb...,
instead of some random interleaving of as and bs. In
practice, cooperative multitasking is sufficient for writing
simple graphical user interfaces.

The yield action allows (forces, in a co-operative multitasking
implementation) a context-switch to any other currently runnable
threads (if any), and is occasionally useful when implementing
concurrency abstractions.

Calling a foreign C procedure (such as getchar) that blocks waiting
for input will block all threads, unless the threadsafe attribute
is used on the foreign call (and your compiler / operating system
supports it). GHC's I/O system uses non-blocking I/O internally to
implement thread-friendly I/O, so calling standard Haskell I/O
functions blocks only the thread making the call.

The threadDelay operation will cause the current thread to
suspend for a given number of microseconds (GHC only).

Note that the resolution
used by the Haskell runtime system's internal timer together with the
fact that the thread may take some time to be rescheduled after the
time has expired, means that the accuracy is more like 1/50 second.

Support for multiple operating system threads and bound threads as described
below is currently only available in the GHC runtime system when the runtime system
has been compiled using a special option.

When recompiling GHC, use ./configure --enable-threaded-rts to enable this.
To find your GHC has already been compiled that way, use
rtsSupportsBoundThreads from GHCi.

Other Haskell systems do not currently support multiple operating system threads.

A bound thread is a haskell thread that is bound to an operating system
thread. While the bound thread is still scheduled by the Haskell run-time
system, the operating system thread takes care of all the foreign calls made
by the bound thread.

To a foreign library, the bound thread will look exactly like an ordinary
operating system thread created using OS functions like pthread_create
or CreateThread.

Bound threads can be created using the forkOS function below. All foreign
exported functions are run in a bound thread (bound to the OS thread that
called the function). Also, the main action of every Haskell program is
run in a bound thread.

Why do we need this? Because if a foreign library is called from a thread
created using forkIO, it won't have access to any thread-local state -
state variables that have specific values for each OS thread
(see POSIX's pthread_key_create or Win32's TlsAlloc). Therefore, some
libraries (OpenGL, for example) will not work from a thread created using
forkIO. They work fine in threads created using forkOS or when called
from main or from a foreign export.

Like forkIO, this sparks off a new thread to run the IO computation passed as the
first argument, and returns the ThreadId of the newly created
thread.

However, forkOS uses operating system-supplied multithreading support to create
a new operating system thread. The new thread is bound, which means that
all foreign calls made by the IO computation are guaranteed to be executed
in this new operating system thread; also, the operating system thread is not
used for any other foreign calls.

This means that you can use all kinds of foreign libraries from this thread
(even those that rely on thread-local state), without the limitations of forkIO.

Run the IO computation passed as the first argument. If the calling thread
is bound, an unbound thread is created temporarily using forkIO.
runInBoundThread doesn't finish until the IO computation finishes.

Use this function only in the rare case that you have actually observed a
performance loss due to the use of bound threads. A program that
doesn't need it's main thread to be bound and makes heavy use of concurrency
(e.g. a web server), might want to wrap it's main action in
runInUnboundThread.

In a standalone GHC program, only the main thread is
required to terminate in order for the process to terminate.
Thus all other forked threads will simply terminate at the same
time as the main thread (the terminology for this kind of
behaviour is "daemonic threads").

If you want the program to wait for child threads to
finish before exiting, you need to program this yourself. A
simple mechanism is to have each child thread write to an
MVar when it completes, and have the main
thread wait on all the MVars before
exiting:

The main thread principle also applies to calls to Haskell from
outside, using foreign export. When the foreign exported
function is invoked, it starts a new main thread, and it returns
when this main thread terminates. If the call causes new
threads to be forked, they may remain in the system after the
foreign exported function has returned.

GHC implements pre-emptive multitasking: the execution of
threads are interleaved in a random fashion. More specifically,
a thread may be pre-empted whenever it allocates some memory,
which unfortunately means that tight loops which do no
allocation tend to lock out other threads (this only seems to
happen with pathalogical benchmark-style code, however).

The rescheduling timer runs on a 20ms granularity by
default, but this may be altered using the
-i<n> RTS option. After a rescheduling
"tick" the running thread is pre-empted as soon as
possible.

One final note: the
aaaabbbb example may not
work too well on GHC (see Scheduling, above), due
to the locking on a Handle. Only one thread
may hold the lock on a Handle at any one
time, so if a reschedule happens while a thread is holding the
lock, the other thread won't be able to run. The upshot is that
the switch from aaaa to
bbbbb happens infrequently. It can be
improved by lowering the reschedule tick period. We also have a
patch that causes a reschedule whenever a thread waiting on a
lock is woken up, but haven't found it to be useful for anything
other than this example :-)